This invention relates to polymers containing alkyl vinylidene cyanide moieties an exhibiting nonlinear optical and piezoelectric properties.
Nonlinear optical activities generally result from interaction of materials with light, and are described in terms of second order nonlinearity, third order nonlinearity, and so on. An introduction to the theory and practical applications of nonlinearity, especially of organic materials, is provided by Nonlinear Optical Properties of Organic Molecules and Crystals, Volumes 1 & 2, edited by D. S. Chemla and J. Zyss, Academic Press, 1987.
It is known that organic small molecules and polymeric materials with large delocalized .pi.-electron systems can exhibit nonlinear optical response, which in many cases is a much larger response than that exhibited by inorganic materials. Examples of such organic small molecules include 2-methyl-4-nitroaniline. Examples of such polymers are described in Nonlinear Optical Properties of Organic and Polymeric Materials, ed. D. J. Williams, ACS Symposium Series No. 233, American Chemical Society, Washington, D.C., 1983. Such materials generally contain in their nonlinear molecular units electron donor groups and acceptor groups linked by a conjugated .pi.-electron unit. This structural pattern gives rise to delocalization of the .pi.-electrons. The delocalized .pi.-electrons are believed to give rise to nonlinear effects when the material interacts with high intensity laser radiation. These effects are manifested as generation of different orders of light frequencies called harmonic frequencies.
While a nonlinear molecule can theoretically generate different orders of harmonic frequencies when it interacts with light, it is generally believed that in order to generate the even numbered harmonic frequencies such as second order, fourth order, and the like, the molecule must possess a "non-centrosymmetric" structure. The non-centrosymmetric structure may be inherent in the molecule or induced externally. A theoretical explanation of non-centrosymmetry and its relationship to harmonic generation can be found in Nonlinear Optical Properties of Organic and Polymeric Materials, referred to above.
Piezoelectricity is the property where electric polarization is produced in a material by application of mechanical strain. Early observations of piezoelectricity were mostly in inorganic crystals such as quartz and Rochelle's salt. However, recently, more and more organic materials, particularly organic polymers, are being shown to possess piezoelectric properties. A historical perspective of the occurrence of piezoelectricity in materials is discussed by P. E. Dunn and S. H. Carr in MRS Bulletin, No. 2 (1989), pages 22-31, published by the Materials Research Society, Pittsburgh, Pa. Other publications of interest in this regard are Electronic Properties of Polymers, by R. Glen Kepler in Treatise on Materials Science and Technology, Volume 10, Edited by J. M. Schultz, Academic Press, New York (1977), pages 670-673; McGraw-Hill Encyclopedia of Science and Technology, Volume 10, pages 216-223, McGraw-Hill Book Company, Inc. (1960); and E. Kolm and H. Kolm, Chemtech, 180 (March 1983).
For both nonlinear optical applications and piezoelectric applications, organic polymers are preferred due to their many advantages. For example, several organic polymers can be cast as thin films by techniques well known in the art. Thin films have the advantage of better utility than single crystals in device fabrication. Furthermore, organic and polymeric materials can be modified structurally to suitably optimize properties such as mechanical stability, thermooxidative stability, and laser damage threshold. Laser damage threshold is an expression of the ability of a material to withstand high intensity laser radiation. The utility of a nonlinear optical material frequently is in a device where the material is subjected to high intensity laser radiation. Unless the material is capable of withstanding such radiation, the device may fail in its intended function.
Polymers which exhibit nonlinear optical activity are described in Nonlinear Optical Properties of Organic and Polymeric Materials, referred to above, as well as, for example in U.S. Pat. Nos. 4,779,961; 4,865,430 and 4,913,844. Optical devices which have a polymeric nonlinear optical component are described in U.S. Pat. Nos. 4,767,169 and 4,865,406. Devices based on optical nonlinearity of materials are described in U.S. Pat. Nos. 3,234,475; 3,395,329; 3,694,055; 4,428,873; 4,515,429; 4,583,818; and by P. W. Smith et al in Applied Physics Letters, 30(6),280 (1977). Devices based on organic materials with conjugated electron systems are described, for example, in U.S. Pat. No. 4,865,406.
Examples of polymers which exhibit piezoelectricity include polyvinyl chloride, polyvinyl fluoride, polyvinylidene fluoride (PVF.sub.2), and polyacrylonitrile. The occurrence of piezoelectricity in these materials has been explained by way of electrets. An electret is an insulator to start with, but generates charged state due to a change in its dipole moment when a DC voltage is applied to it. Typically, in the case of piezoelectric polymers, this charged state is achieved by stretching the polymer film and maintaining the film at a high temperature when the DC voltage is applied. The film is then cooled "freezing in" the charged state.
Polymeric electret elements are useful in electro-acoustic conversion devices, electro-mechanical conversion devices, pressure-sensitive elements, bimorph elements, microwave detection devices, image-recording light-sensitive elements, and the like. Some of the devices based on piezoelectric polymers are described by J. V. Chatigny and L. E. Robb in Sensors, 6 (May 1986) and by P. L. Squire in Sensors, 12 (July 1986).
There is a continuing research effort to develop new nonlinear optical and piezoelectric organic systems for prospective novel phenomena and devices.
Among the recent materials that have been studied for their nonlinear optical properties and/or piezoelectric properties, polymers of vinylidene cyanide have received a lot of attention. For example, U.S. Pat. Nos. 2,615,868 and 4,591,465 describe copolymers of vinylidene cyanide. Copending patent application Ser. No. 491,138 filed on Mar. 9, 1990, now U.S. Pat. No. 5,057,588 describes copolymers of vinylidene cyanide; copending patent application Ser. No. 570,064 filed on May 20, 1990, now U.S. Pat. No. 5,061,764 describes copolymers of vinylidene cyanide which exhibit nonlinear optical and piezoelectric properties. S. Miyata et al, Polymer Journal, Vol. 12, page 857 (1980) describe an alternating copolymer of vinylidene cyanide and vinyl acetate that exhibits piezoelectric properties.
While vinylidene cyanide copolymers are known to possess nonlinear optical and piezoelectric properties, there are several disadvantages with those polymers, especially related to their synthesis. The monomer, vinylidene cyanide, is difficult to synthesize. High temperature pyrolytic conditions are employed for the preparation of the monomer, which then has to be handled under special conditions. It is hydrolytically unstable. Storage of the monomer is, therefore, difficult. The quantity of monomer that can be prepared and stored at any given time is consequently limited, and therefore, the quantity of polymers that can be prepared is also limited. Because of the instability of the monomer, preparation of the polymers necessitates employing special and very stringent conditions. This instability of the monomer is also reflected in the homopolymer of vinylidene cyanide. S. Miyata etal, Polymer Journal referred to above report that poly(vinylidene cyanide) is so unstable that it undergoes scission by atmospheric moisture at room temperature.
Therefore, there has been a continuing need to make polymers with interesting and improved nonlinear optical and piezoelectric properties from monomers that are readily available or readily synthesizable, and are also stable under ordinary conditions. Such monomers also must be easily polymerizable to make homopolymers as well as copolymerizable with other comonomers. The polymers so obtained should be stable under ordinary conditions, and easily castable as amorphous films, which then may be fabricated into devices.
Accordingly, it is an object of this invention to prepare novel polymers that exhibit interesting nonlinear optical and piezoelectric properties.
It is yet another object of this invention to provide amorphous films made from the above novel polymers.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.